Soil fertility

the factors that add to soil productivity

Farmers have known for a very long time that
certain substances (such as dung and ash), when added to the soil, improve
production. These are now called fertilisers. For reasons of cost and ease
of use, chemical fertilisers have replaced natural ones. Although plants
can't distinguish the difference, artificial fertilisers can easily be
over-used, resulting in damage to the soil, rivers and ocean. Learn to
know how to produce more, while damaging the soil and environment less.
What do plants need, how are nutrients formed and maintained and what can
we do to increase the natural fertility of the soil?

A plant's most important need is water. In most
places on Earth, water is a problem. There is either too much of it or
too little. Water is needed by soil organisms too, so a farmer's most urgent
task is to manage the supply of water.

Nutrients are found in the rocks. Once weathered
into soil, these become available to plants. This supply is not enough,
the reason why all terrestrial ecosystems recycle their nutrients with
minimal
losses. Agricultural soil should recycle its nutrients too, but there are
insurmountable problems.

Because plants do not need salt the way animals
and humans do, salt is easily lost from our soils, particularly through
modern farming. Produce does not only taste weak, it also contains fewer
salts. Salt deficiency in society may arrise from other causes too.

links

NPKnowledge:
Iowa State University publications about fertilising and soil management.

Plant needsIn the chapter on geology, we've seen that the base rocks from which
soil is weathered, ends up quite different in composition from place to
place, but in practice all fertile soils on earth follow the rather constant
chemical composition of plants, which is similar to animals. This can be
understood from the way plants, animals and soil form an ecosystem, cycling
the available nutrients many times before they get lost. In the process,
unnecessary concentrations of elements (like salt and chlorine) do get
lost, resulting in concentrations of available soil elements, closely matching
plants' needs, everywhere on Earth. Although their ratios of elements are
similar, soils may vary considerably as to their densities, and thus fertility.

Underground, the soil nutrients are not kept in solution but inside
the bodies of the living organisms (and some are adsorbed onto clay platelets).
No wonder then that the amount of life in good soils is 2 to 10 times more
than that above ground. The nutrients become available when some organisms
die, which happens frequently because they grow fast. But it does not happen
in sudden boosts, as is needed for a monoculture that has been planted
all at the same time, like a potato crop. In this respect, natural, productive
soil appears to need more fertiliser than it actually does. Modern farming,
driven by economic constraints, is forced to use artificial fertilisers,
often to the detriment of the soil's natural fertility.

The ecologist Edward S Deevey Jr discovered that living matter
consists mainly of the six elements hydrogen (H), oxygen (O), carbon (C),
nitrogen (N), phosphorus (P) and sulphur (S), in the ratio H:O:C:N:P:S
= 2960:1480:1480:16:1.8:1, which is an average for all living organisms
on Earth. Of these, the woody plants far outnumber all others, so the formula
is biased towards these. The ratio H:O:C:N:P:S = 1600:800:800:9:1:5 is
often used for land plants, and 212:106:106:16:1:2 for marine plants and
soil humus. From an ecological perspective, it would not be surprising
if scientists discover that these ratios for terrestrial life (green matter
in plants + animals) are the same as for soil biota (bacteria + fungi +
animals). By comparison, the most common component of plants are
the carbohydrates (sugars, starches and woody substances), represented
by H:O:C = 12:6:6 atoms, or as masses 1:6:8.
With C, O and N having similar atomic masses (12, 16, 14), as a rule
of thumb, each unit of nitrogen belongs to 200 units of life (dried) and
100 units of carbon.

oxygen: when plants rest at night, they need oxygen, while producing
carbon dioxide. In slow-growth areas such as the Boreal forests, respiration
during the long winters is almost equal to photosynthesis during the short
summers.

warmth: to be able to perform the biochemical processes of life.
Plants have adapted to a wide range of temperature, but the warmth of the
tropics promotes highest productivity.

water: the biochemical process of photosynthesis requires much water.
Water or the lack of it, causes problems in most geographic areas.

macronutrients: the main nutrients N,P,K,S,Ca,Mg and micronutrients,
the trace elements.

The soil biota have similar requirements, but since they do not photosynthesise,
they need neither light nor carbon dioxide. The requirements above are
often called 'limiting factors' because each could limit the plant's growth.
More accurately, they should be called 'life-determining factors'.

Liebig's LawThe scientist Liebig discovered that all of the above needs need to
be satisfied, and that the one in shortest supply will be the main cause
of limiting growth. Thus in winter, when it freezes, plants do not need
either carbon dioxide or water or nutrients. What they need is warmth first.

Sunlight and warmthSunlight and warmth go together, since the only input of energy comes
from the sun (see oceanography/radiation).
Seasonal cycles affect particularly the temperate areas. But it can be
influenced considerably. A glasshouse for instance, traps heat radiation
by trapping visible light but preventing infrared radiation from escaping.
In cold climates, glasshouses are often heated by burning fossil fuel.
Cropland can be sheltered from cold winds, by means of shelter belts. Heat
from sunlight can be trapped by stands of vegetation. Evaporation from
soil causes enormous loss of warmth, but it can be minimised by mulching
or planting a soil-covering crop.

The amount of sunlight in summer may be too much, causing the soil to
dry out. Sheltering trees can be planted that lose their leaves in winter.
Crops can be spaced properly to prevent them shading each other out.

Carbon dioxideCarbon dioxide is rather scarce in our atmosphere, where it is found
as one molecule in every 30,000. All plants on the planet compete for this
resource, since all places on earth connect to the same atmospheric pool
of carbon dioxide. The most successful plants, living in warm tropical
areas scavenge it more successfully than plants living in cool areas with
less light.
Only recently did nature evolve a plant, capable of converting carbon
dioxide more efficiently than any other plant, while also using less water.
Their photosynthetic conversion requires four biochemical steps, rather
than the usual three, a process that saves it both energy and water. These
plants, called C4 plants, include the bamboo-like grasses, and the
agricultural crops sugarcane, maize and sorghum. They are about twice as
efficient in converting sunlight and need four times less water. C3 plants
have maximum sunlight conversion efficiency of 15% and C4 grasses up to
24%. In practice, due to leaf shading, these figures are five times lower.
Photosynthesis in C3 plants converts 0.1-0.4 g CO2 with 1 kg water, whereas
C4 plants convert 0.4-0.8 gram.

Succulent plants are active at night, taking up CO2 with their stomata
(leaf pores) wide open, when other plants close theirs to minimise respiration.
During the night, CO2 is absorbed and converted into chemical storage as
oxaloacetic acid and then as malate. During the day, these compounds are
converted and normal C3 photosynthesis takes place, with the plant's leaf
pores closed to prevent unnecessary evaporation. This special form of CO2
fixation is called Crassulean Acid Metabolism (CAM). CAM plants
are succulents, agaves, lilies, bromeliads, orchids, cacti, euphorbia,
geraniums and many more. They use a minimum of water. (For more details
and differences between C3, C4 and CAM plants, see the table
below)

As can be expected, the C3 plants, which are limited in their CO2 uptake,
react more vigorously to CO2 increases than the C4 plants. They also still
outnumber the C4 plants, which are limited by temperature.

In externally heated glasshouses, carbondioxide from burnt fossil fuel
for heating, is often piped into the glasshouse to enhance growth.

Water and nutrients will be discussed
in their own subchapters below. See also the periodic
table of elements for essential nutrient needs and symptoms of deficiency
in plants, animals and humans.

WateringWater is by far the most restrictive of a plant's needs. In spite of
the massive size of the water cycle, which causes
rain and snowfall, water is in short supply in most areas of the world,
at least during one or more seasons. Water is not only necessary for a
plant's survival but also for its soil biota, on which it ultimately depends.
Likewise, the success of farming, depends mainly on how to keep the underground
'circus' alive, and with it, the above ground vegetation.

Plants need water, a large amount of it when growing. The table below
gives an indication of how much water is transpired to produce one kg of
dry matter.

Average
transpiration ratios for various plant typesWater amounts in kg per
kg dry matter (transpiration ratio).

A hectare of highly productive grain produces 8 ton of grain and some
10 ton dry matter, requiring some 10 million litres of water during the
season (4 months) for photosynthesis alone, or 100,000 litres per day,
or 1000mm of rain!
It is common sense therefore, to irrigate crops for higher productivity,
and also to increase the cropping area. Particularly as an insurance against
the vagaries of weather and climate, farmers all over the world are tapping
whatever water resources they can find. The most common of these are ground
water and artificial lakes.

Ground water and aquifersAlthough soil and rock are compressed by tremendous forces, there are
nonetheless gaps and cracks that have been interconnected by flowing water.
One would have expected that water, being three times lighter than rock,
is pushed up as sediments and rocks are pushed down by their own weight,
so that free water cannot exist at depth. However, as can be observed in
limestone Karst systems, water can exist deep down to 300 m and perhaps
even deeper. What's more, these underground aquifers are interconnected
as if it were a single underground lake, accessible by all who live above
it.

Pumping groundwater aquifers is so attractive because the water does
not need to be transported. But aquifers replenish slowly. The deeper they
are, the longer it takes. Saudi Arabia is estimated to have some 2000 cubic
km of 10,000 - 30,000 year old water stored in aquifers to 300m deep.

The
Ogallala aquifer in the USA spans eight states, covering some 452,000 square
km, and estimated to hold 3700 cubic km of water, a volume equal to the
annual flow of more than 200 Colorado rivers, an underground 'lake' of
120m deep. Today, the Ogallala alone, waters 20% of US irrigated land,
depleting it by 12 cukm/yr. In several decades of pumping, the 3700 cukm
reservoir has been shrunk by 325 cukm, facing extinction 300 years from
now. It is the typical tale of all ground water reservoirs in the world.

Bangladesh is sinking into the sea because its groundwater has been
pumped so extensively. In other places the lower water table is drying
out valuable wetland areas. One may think that it is a stupid idea to pump
water from underneath the plant's roots in order to put it on top of the
land, where much of it evaporates. Yet this is exactly what has been happening
all over the world. Since the groundwater is used by all but owned by none,
it follows the 'tragic of the commons' (why should I limit my use, when
the other guy is not?), unless rigorously managed by governments.

Groundwater is formed from water penetrating the soil and sinking to
deeper levels. As it is pumped, the water table drops, encouraging water
to flow more freely and thereby carrying substances that should not be
there. The table below gives an idea of the kinds of threats to groundwater
systems and how these affect humans. Note that the effects on the environment
are not mentioned.

Chemical
threats to groundwater

threat

source

effects

where

pesticides

runoff from farms, backyards,
golf courses, landfills

organochlorides linked to
reproductive and endocrine damage in wildlife; organophosphates and carbamates
linked to liver and nervous system damage and cancers.

USA, eastern Europe, China,
India

nitrates

fertiliser runoff; manure
from livestock operations; septic systems.

restricts amount of oxygen
reaching brain, which can cause death in infants (blue baby syndrome).

Mid-Atlantic USA, north
China plain, western Europe, northern India.

petro-chemicals

underground petroleum storage
tanks

benzene and other petrochemicals
can cause cancer, even at low exposure

Irrigation from artificial lakesAbout 6000 years ago the Sumerians invented irrigation by diverting
water from the Euphrates river to their crop lands. It improved yield and
living conditions considerably. Today, wherever feasible, rivers are dammed
for hydro electricity and irrigation. The high water pressure makes it
possible to transport high volumes of water through a system of reticulated
pipes. When carefully managed, it allows farmers to extend their cropping
season and to increase productivity. One would think that irrigation is
just another form of rainfall, but it is not.

The water collecting in a reservoir is the run-off from rain falling
on the upper-catchment area. In its journey to the lake, it has dissolved
valuable nutrients but also the not so valuable salts that have been discarded
by living soil. If this water were applied to soils that experience a good
soaking several times per year, the salts would be washed further down
the slopes, eventually ending in the sea. But so often it is the irrigated
land's main source of water. As water evaporates from the soil, it leaves
the salts behind, resulting in gradual salinisation which degrades the
land. Much irrigated cropland has been lost this way. As stated before,
it is difficult (or risky) to bring dry land into production. Irrigation
from lakes can help in some climate situations, mainly to reduce the risk
of drought. Hydro lakes do reduce the flow of the river, resulting in less
flooding downstream and thus less soil fertility replenishment. The Aswan
Dam in Egypt has caused such problems.

The table below shows how much world agriculture depends on irrigation
of its crops. Not surprisingly, the driest countries rely on it the most,
and it is in these places that irrigation brings its problems. In the table
below, padi culture has been included as irrigated land, but this is a
sustainable form of water harvesting. The growth of irrigated cropland
first kept pace with world population growth, but is now falling behind,
mainly because the most suitable land has been used. About 20% of irrigated
land is damaged by salinisation.

Water harvestingHaving a water lake above each farm sounds like a good idea. The stored
water can reach lower farmland through the water table or by being piped
there. Small lakes or ponds are used in this way to provide for drinking
water for grazing stock, but the larger lakes are too much of an engineering
challenge.

One sound ecological way is to leave a stand of forest above each farm,
crowning the hill tops. Forests can soak up large quantities of water and
release these slowly down-slope. Hill tops are difficult to farm because
of their low water tables, but they are relatively flat, offering access
to tractors, a reason why many have been denuded. But in Japan, steep hillsides
and hill tops have been left alone, clad in their native forests.

Water savingWater can be saved by reducing evaporation direct from the soil. Water
evaporates faster in high temperatures and wind. So if wind speed can be
reduced at soil level (and above it) while the soil can be kept cool, much
water loss can be avoided. Covering the soil with mulch and erecting wind
break hedges is one solution. In Spain and around the Mediterranean Sea,
where the climate is too dry in summer, farmers till the soil under their
olive trees to prevent weeds drawing water and mulch the soil with tilled,
dry soil. However, this method leaves the soil wide open to erosion when
sudden rains appear.

Irrigation through open and unpaved channels, and applying it to the
land through surface furrows, may lose 50% of the water into the soil where
it is not needed and through evaporation. Applying water to crops by means
of drip irrigation, although more expensive, can reach up to 95% efficiency
in water use. Water savings have been achieved by replacing high pressure
sprinklers that make fine droplets, with low pressure sprinklers making
large droplets.
In many places in the world, fresh water is now a commodity that can
be traded in the freemarket. With the aim of encouraging farmers to conserve
water, it has also opened the way to feudal land ownership and water rights
being bought by industries and cities, who are in a better position to
offer higher bids.

Water horror stories

United States: The High Plains Aquifer System (Ogallala) underlies
20% of all US irrigated land and contains some 3700 cukm. Net depletion
in 30 years amounts to 325 cukm. More than 65% of this depletion has occurred
in the Texas High Plains, where irrigated area dropped by 26% between 1979
and 1989. Current depletion is estimated at 12 cukm/yr.

United States, California: Groundwater overdraft averages 1.6 cukm/yr,
amounting to 15% of the state's annual net groundwater use. Two thirds
of the depletion occurs in the Central Valley, the country's (and to some
extent the world's) fruit and vegetable basket.

United States, Southwest: Overpumping in Arizona alone totals more
than 1.2 cukm/yr. East of Phoenix, water tables have dropped more than
120m. Projections for Albuquerque show that, if groundwater withdrawals
continue at current rates, water tables will drop an additional 20m by
2020.

Mexico City and Valley of Mexico: Pumping exceeds natural recharge
by 50-80%, which has led to falling water tables, aquifer compaction, land
subsidence, and damage to surface structures.

Arabian Peninsula: Groundwater use is nearly three times greater
than recharge. Saudi Arabia depends on nonrenewable groundwater for roughly
75% of its water, which includes irrigating 2-4 Mt/yr wheat. At this depletion
rate, groundwater reserves would last only about 50 years.

North Africa: Net depletion of groundwater in Libya totals nearly
3.8 cukm/yr. For the whole of North Africa, current depletion is estimated
at 10 cukm/yr.

Israel and Gaza: Pumping from the coastal plain aquifer bordering
the Mediterranean Sea exceeds recharge by some 60%. Salt water has invaded
the aquifer.

Spain: One-fifth of total groundwater use, or 1 cukm/yr, is unsustainable.

India: Water tables in the Punjab, India's bread basket, are falling
0.2m annually across two-thirds of the state. In Gularat, groundwater levels
declined in 90% of observation wells monitored during the 1980s. Large
drops have also occurred in Tamil Nadu.

North china: The water table beneath portions of Beijing has dropped
37m over the last 4 decades. Overdraft is widespread in the north China
plain, an important grain-producing region.

Southeast Asia: Significant overdraft has occurred in and around
Bangkok, Manila and Jakarta. Overpumping has caused land to subside beneath
Bangkok at a rate of 5-10 cm/yr for the past two decades.

It is evident that practically everywhere on Earth, the amount of irrigation
water is seriously overdrawn. Prospects for increasing agricultural yield
are therefore not optimistic. It is not only land that needs water in large
volumes, but also industries and people. As the world's population grows
and becomes urbanised (where else are jobs found?), water may shift to
where it is valued more, the industries and cities. In 25 years, India
will add some 340 million people to its cities, more than the current population
of the USA and Canada combined. Saving water is not just an agricultural
problem, but should be achieved in cities as well.

This
diagram shows how the world's fresh water resources are heading for a climax.
Net fresh water falling on the land is about 40,000 cubic km/yr. Most of
this runs off in floods and won't penetrate the soil. Some of the flood
water is caught in dams (green area), which increases both the base flow
and the amount of accessible water. Back in 1950, human consumption was
only a fraction of accessible water, but by 1950 it became 50% and by the
end of the millennium it stood at 80%. Only rapid building of dams can
prevent total human demand from catching up with the amount of accessible
water, but this can no longer be achieved. As a result, there will be a
world-wide shortage of drinking and industrial water after the year 2020.